The present invention relates to a low-profile printed circuit board and, more particularly, relates to such a printed circuit board including planar magnetic elements, such as transformers, inductors, and like components.
Recent advances have been made in designing and building low profile printed circuit boards. In the quest for reliable, miniaturized, low profile circuits built on insulating substrates such as plastics and ceramics, there has been a shift from high profile components with solderable leads to chip mounted components. With the advent of thick-film technology and advanced etching techniques, there has been a further shift toward screen-printing or etching components and circuits such as resistors, capacitors and circuit conductor paths directly on a substrate. Such components are not only extremely low-profile, but also have proven to be temperature and vibration stable, repeatable and cost effective.
Although there are advantages associated with the printing or etching of components and circuits, it has been difficult to produce planar magnetic elements such as inductors and transformers by these methods. One reason for this difficulty is the limited surface area available on the substrate to accommodate the two-dimensional windings required to construct planar magnetic elements. Moreover, since the windings are printed in a roughly spiral shape, the inner terminus or end of the winding is surrounded by the outer turns of the spiral. Thus, a mechanical bridge together with possible solder connections or a multi-layer circuit is needed to connect the inner end of the winding with other circuitry on the substrate. Difficulties also arise in insulating the individual two-dimensional windings relative to each other, which are often stacked to form a planar magnetic element such as, for example, a transformer, and in insulating the windings relative to respective ferrite cores, such as in a transformer or inductor structure.
There have been attempts to overcome the various difficulties associated with printed circuit boards incorporating planar magnetic elements. U.S. Pat. No. 4,622,627 discloses a planar magnetic transformer which includes a primary winding formed on a first substrate and a secondary winding located on the same or opposite face of the first substrate or on a second substrate. Top and bottom inductive cores are connected together through apertures in the substrate(s) and magnetically coupled with the windings. While it is possible to achieve a low-profile transformer in this manner, there remains the problem of the inner terminus of the winding being surrounded by the outer turns of the spiral. Accordingly, a mechanical connection or additional dielectric and conductive layers are necessary to connect the inner terminus of the winding with other circuitry on the substrate. In addition to these problems, valuable substrate surface area is sacrificed where both the primary and secondary windings of the transformer are placed on the same substrate.
U.S. Pat. No. 4,543,553 attempts to overcome the problem of limited surface area by stacking double-sided magnetic layers on top of one another to simulate a three-dimensional coil. Each of the double-sided magnetic layers contains a single, printed turn of a rectangular coil, and the layers are staggered in relation to one another so that both sides of each layer add turns to the coil. Corresponding conductive turns on the upper and lower sides of selected magnetic layers are electrically connected and mechanically adhered to one another by heating, crimping and sintering the layers via through-holes provided in a middle-layer positioned between the top and bottom layers. Thus, a laminated structure is formed, and electrodes are subsequently placed on opposite ends of the laminated structure in order to form a chip inductor. Although the above-mentioned process uses printed conductor turns, the advantages of a low-profile device having a two-dimensional screen-printed or etched winding are lost through stacking of the magnetic layers. Moreover, the process for electrically and mechanically connecting the coil turns and the need for additional electrodes adds expense and makes fabrication of the chip inductor difficult.
U.S. Pat. No. 5,010,314 also discloses planar magnetic elements. The patent teaches two flat bobbin trays each having a raised rectangular guide at its center. The trays house substrates supporting the primary and secondary spiral conductive patterns of a transformer. Layers of dielectric sheet material are provided between the substrate layers supporting the windings as well as between the two bobbin trays. Top and bottom inductive cores extend through an aperture formed in the center of the bobbins, the substrates and the dielectric layers so as to magnetically couple the planar windings to the core. Connections between different conductive layers are established by solder leads that extend beyond the perimeter of the laminated structure. The use of bobbins in the configuration eliminates the advantages associated with a low-profile, planar element. Moreover, the plurality of layers needed to form the element and the solder leads required for connections between layers make the transformer both expensive and difficult to fabricate.
Another prior approach to constructing planar conductive circuits, including planar windings, has been to encapsulate the windings in sheets of polyamide, which has been commercially available under the trade name "Kapton". In one such device made by Tech-Etch of Plymouth, Mass., a circular, planar winding was encapsulated within the polyamide, with the polyamide forming margins along the inner and outer perimeters of the winding. The winding was not surface mounted on a substrate, but rather was encapsulated by itself within the polyamide forming a relatively flexible winding structure. One drawback with this approach is that the polyamide insulating material is quite expensive, and virtually cost prohibitive for commercial electronic applications. Moreover, the polyamide is relatively difficult and expensive to work with. For example, because of the shear strength of the material, a heavy-tonnage press is required in order to cut the polyamide sheets cleanly and in desired patterns necessary for printed circuit applications.
Accordingly, it is a general object of the present invention to provide a low-profile printed circuit board which includes at least one planar magnetic element formed integrally with other circuitry carried on the board.
It is another object of the invention to provide a planar magnetic element which does not require mechanical connections to connect windings or portions of windings with one another or with other circuitry on the printed circuit board.
It is another object of the present invention to electrically insulate the windings or portions of the windings comprising such a planar magnetic element from one another and/or from respective conductive cores without the need for bobbins or the expensive dielectric layers used in the past.
It is a further object of the present invention to provide additional substrates or printed circuit boards including one or more second planar magnetic windings, such as secondary transformer windings and/or coupled inductor windings, for surface mounting onto the primary circuit board so that each second winding is overlying a respective first winding on the primary board for creating a multiple output device.
These and other objects and advantages of the present invention will become apparent in view of the following summary, detailed description and accompanying drawings of the present invention.